Split out specification documentation, prepare for better configuration overides.

doc/calendar.md

@@ -118,6 +118,13 @@ granularity. The period contains fields ```since``` which is the first
 month of the period and ```before``` which is the first month just
 after the period.
 
+    (*current-month* [month])
+
+* ```month``` - valid month structure as specified in the ```month``` module
+
+Configuration parameter specifying the current month. Defaults to the
+current month derived from the current system time.
+
     (period-since p)
 
 * ```p``` - valid period